Contract Assets and Contract Liabilities - The following tables set forth the changes in our contract asset and contract liability balances for the periods indicated:



Contract Assets	(Millions of dollars)
Balance at January 1, 2018 (a)	$	6.4
Amounts invoiced in excess of revenue recognized	(0.9		)
Net additions	0.7
Balance at December 31, 2018 (b)	6.2
Amounts invoiced in excess of revenue recognized	(1.7		)
Net additions	0.5
Balance at December 31, 2019 (c)	$	5.0

(b) - Contract assets of $1.7 million and $4.5 million are included in other current assets and other assets, respectively, in our Consolidated Balance Sheet.

(c) - Contract assets of $1.3 million and $3.7 million are included in other current assets and other assets, respectively, in our Consolidated Balance Sheet.



Contract Liabilities	(Millions of dollars)
Balance at January 1, 2018 (a)	$	33.3
Revenue recognized included in beginning balance	(19.5		)
Net additions	17.9
Balance at December 31, 2018 (b)	31.7
Revenue recognized included in beginning balance	(15.6		)
Net additions	41.0
Balance at December 31, 2019 (c)	$	57.1

(b) - Contract liabilities of $15.6 million and $16.1 million are included in other current liabilities and other deferred credits, respectively, in our Consolidated Balance Sheet.

(c) - Contract liabilities of $22.2 million and $34.9 million are included in other current liabilities and other deferred credits, respectively, in our Consolidated Balance Sheet.

In 2019, net additions for contract liabilities relate primarily to deferred revenue on contributions in aid of construction received from customers and NGL storage contracts.

Receivables from Customers and Revenue Disaggregation - Substantially all of the balances in accounts receivable on our Consolidated Balance Sheets at December 31, 2019, and December 31, 2018, relate to customer receivables. Revenues sources are disaggregated in Note Q.

Practical Expedients - We do not disclose the value of unsatisfied performance obligations for (i) contracts with an original expected length of one year or less and (ii) variable consideration on contracts for which we recognize revenue at the amount to which we have the right to invoice for services performed.

Transaction Price Allocated to Unsatisfied Performance Obligations - The following table presents aggregate value allocated to unsatisfied performance obligations as of December 31, 2019, and the amounts we expect to recognize in revenue in future periods, related primarily to firm transportation and storage contracts with remaining contract terms ranging from one month to 24 years:



Expected Period of Recognition in Revenue	(Millions of dollars)
2020	$	343.5
2021	290.4
2022	214.8
2023	166.4
2024 and beyond	807.2
Total estimated transaction price allocated to unsatisfied performance obligations	$	1,822.3

The table above excludes variable consideration allocated entirely to wholly unsatisfied performance obligations, wholly unsatisfied promises to transfer distinct goods or services that are part of a single performance obligation and consideration we determine to be fully constrained. Information on the nature of the variable consideration excluded and the nature of the

performance obligations to which the variable consideration relates can be found in the description of the major contract types discussed in Note A. The amounts we determined to be fully constrained relate to future sales obligations under long-term sales contracts where the transaction price is not known and minimum volume agreements, which we consider to be fully constrained until invoiced.